The invention relates to an insulation monitoring device of a direct current power supply system in operation and insulated from the ground, having an alternating current generator connected between the ground and the supply system to apply an alternating current to the supply system at a voltage that is not dangerous for people, and a current transformer having a measuring toroid bearing a primary winding constituted by the conductor of said supply system and a secondary measurement winding.
Direct current power supply systems are frequently used in motor supply and control stations or on boats, and these installations do not tolerate power failures The system is insulated from the ground and a first ground fault therefore does not hinder correct operation of the installation. It is nevertheless imperative that this first fault be detected, as it may degenerate or generate a short-circuit when a second ground fault occurs. Monitoring is preferably continuous and in all cases must not involve the power supply system being put out of operation. Monitoring at various points of the supply system moreover enables the fault to be rapidly located. The first ground fault induces an alternating current to flow through the system and the ground, but the intensity of this current, which depends on the voltage of the alternating current generator and on the fault circuit resistance, is for safety reasons relatively low, for example from 3 to 20 mA, compared to the direct current intensity which may reach several hundreds of Amps. Under these conditions, detection of the alternating component of the current in the system cannot be achieved by standard current transformers, the magnetic material of the toroid of which is saturated by the direct current.
Using another type of sensor, for example a Hall-effect sensor, which is not liable to be saturated, does not give satisfaction, as their measuring precision is insufficient.
The object of the invention is to achieve a reliable insulation monitoring device that is not dangerous for people.